1. Field of the Invention
The present invention relates to a system for monitoring the span loss of a transmission line in a WDM (Wavelength Division Multiplex) transmission apparatus.
2. Description of the Related Art
In a WDM system, it is necessary to adjust the input level of a received-light amplifier to be within an operation range or to detect level fluctuation due to a transmission line, which influences a main signal, at the time of initial construction or at the time of switching the transmission line after start of operation, and, therefore, it is important to measure and monitor the span loss of a relevant transmission line.
Since the input/output level always fluctuates within a certain range of width, it is not possible to measure correct span loss unless maintainers of both the upstream-station apparatus and downstream-station apparatus measure and compare the level at the same timing. Therefore, in order to measure span loss, a procedure is generally taken in which the maintainers enter the upstream station and the downstream station, confirm the output level of the upstream-station apparatus and the input level of the downstream-station apparatus, respectively, with a level monitoring function implemented for measuring instruments or the apparatuses and manually compare the levels. When a main signal is influenced by level fluctuation of the transmission line being used, the upstream-station apparatus and the downstream-station apparatus read out level monitor values, respectively, and manually compare the values to confirm the influence.
There is proposed an invention for controlling the output level of an optical amplifier on the receiving side of a WDM system (for example, see JP2007-104103A). The WDM system of JP2007-104103A is provided with a sending-side optical transmission apparatus and a receiving-side optical transmission apparatus, and it operates as shown below. The sending-side optical transmission apparatus sends the-number-of-wavelengths information as regards the signal light to the receiving-side optical transmission apparatus using an OSC (Optical Supervisory Channel). The receiving-side optical transmission apparatus detects a transmission line abnormality or an increase/decrease in the number of wavelengths multiplexed on a WDM light using the received OSC light and adjusts the amount of attenuation of an optical signal at the time of receiving the optical signal in accordance with a result of the detection.
As described above, confirmation of span loss requires much time and effort from personnel because comparison between an upstream station and a downstream station is manually performed. Therefore, there is a demand for simplification thereof, and accurate monitoring is a problem to be solved.
In the WDM system of JP2007-104103A, the timing for measuring the level of an optical signal is not considered. A configuration for measuring the levels of optical signals at the upstream station and the downstream station at the same hour is not disclosed. Therefore, the WDM system of JP2007-104103A cannot measure the levels of the optical signals at the upstream station and the downstream station at the same hour and, consequently, cannot accurately measure span loss.